Re: [RFC] Tracking mlocked pages and moving them off the LRU

[Christoph Hellwig]

On Fri, Feb 02, 2007 at 10:20:12PM -0800, Christoph Lameter wrote:
> This is a new variation on the earlier RFC for tracking mlocked pages.
> We now mark a mlocked page with a bit in the page flags and remove
> them from the LRU. Pages get moved back when no vma that references
> the page has VM_LOCKED set anymore.
> 
> This means that vmscan no longer uselessly cycles over large amounts
> of mlocked memory should someone attempt to mlock large amounts of
> memory (may even result in a livelock on large systems).
> 
> Synchronization is build around state changes of the PageMlocked bit.
> The NR_MLOCK counter is incremented and decremented based on
> state transitions of PageMlocked. So the count is accurate.
> 
> There is still some unfinished business:
> 
> 1. We use the 21st page flag and we only have 20 on 32 bit NUMA platforms.
> 
> 2. Since mlocked pages are now off the LRU page migration will no longer
>    move them.
> 
> 3. Use NR_MLOCK to tune various VM behaviors so that the VM does not 
>    longer fall due to too many mlocked pages in certain areas.

This patch seems to not handle the cases where more than one process mlocks
a page and you really need a pincount in the page to not release it before
all processes have munlock it or died.  I did a similar patch a while
ago and tried to handle it by overloading the lru lists pointers with
a pincount, but at some point I gave up because I couldn't get that part
right.

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